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(Overriding Makefiles): Don't suggest using .DEFAULT; that loses if the

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target file exists.  Instead recommend the combination of a match-anything
rule and a force target.
This commit is contained in:
Roland McGrath 1994-05-11 01:42:43 +00:00
parent e83c7d40cd
commit c25db14264
1 changed files with 17 additions and 9 deletions
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@ -1247,13 +1247,12 @@ include one in the other, and add more targets or variable definitions.
However, if the two makefiles give different commands for the same
target, @code{make} will not let you just do this. But there is another way.
@cindex @code{.DEFAULT}, used to override
@cindex match-anything rule, used to override
In the containing makefile (the one that wants to include the other),
you can use the @code{.DEFAULT} special target to say that to remake
any target that cannot be made from the information in the containing
makefile, @code{make} should look in another makefile.
@xref{Last Resort, , Defining Last-Resort Default Rules},
for more information on @code{.DEFAULT}.
you can use a match-anything pattern rule to say that to remake any
target that cannot be made from the information in the containing
makefile, @code{make} should look in another makefile.
@xref{Pattern Rules}, for more information on pattern rules.
For example, if you have a makefile called @file{Makefile} that says how
to make the target @samp{foo} (and other targets), you can write a
@ -1263,18 +1262,27 @@ makefile called @file{GNUmakefile} that contains:
foo:
frobnicate > foo
.DEFAULT:
%: force
@@$(MAKE) -f Makefile $@@
force: ;
@end example
If you say @samp{make foo}, @code{make} will find @file{GNUmakefile},
read it, and see that to make @file{foo}, it needs to run the command
@samp{frobnicate > foo}. If you say @samp{make bar}, @code{make} will
find no way to make @file{bar} in @file{GNUmakefile}, so it will use the
commands from @code{.DEFAULT}: @samp{make -f Makefile bar}. If
commands from the pattern rule: @samp{make -f Makefile bar}. If
@file{Makefile} provides a rule for updating @file{bar}, @code{make}
will apply the rule. And likewise for any other target that
@file{GNUmakefile} does not say how to make.@refill
@file{GNUmakefile} does not say how to make.
The way this works is that the pattern rule has a pattern of just
@samp{%}, so it matches any target whatever. The rule specifies a
dependency @file{force}, to guarantee that the commands will be run even
if the target file already exists. We give @file{force} target empty
commands to prevent @code{make} from searching for an implicit rule to
build it---otherwise it would apply the same match-anything rule to
@file{force} itself and create a dependency loop!
@node Rules, Commands, Makefiles, Top
@chapter Writing Rules